Which of the following is an example of gene flow?

When the genes propagate from one area to another. 
The founder effect happens when genetic diversity declines due to an external larger group of organism invades a set of current living population. An example of this process is when the British colonies invaded the Mesoamerican continent where native americans lived and were outrun by the British colones, especially since many wars were made and ethnic cleansing. 

The answer is option a